Magnetic random access memory (MRAM) is memory using an MTJ element 1, the basic configuration of which is a layered structure of a ferromagnetic free layer 2/insulating layer 3/ferromagnetic fixed layer 4, as shown in FIG. 2. In an MTJ element 1, a phenomenon (the tunnel magnetoresistance or TMR effect) is observed in which the resistance value for tunnel current flowing in the perpendicular direction between layers is different according to whether the direction of the magnetizations of the set of the ferromagnetic free layer 2 and the ferromagnetic fixed layer 4 are parallel or antiparallel. Thus by applying this phenomenon, binary data, recorded as two states which are parallel and antiparallel sets of magnetization directions, can be read out. Moreover, the magnetization directions recorded in the respective ferromagnetic members of the ferromagnetic free layer 2 and the ferromagnetic fixed layer 4 are retained even when the current is turned off. MRAM applies these readout operations and storage/retention operations to nonvolatile memory. Throughout the Specification of this application, descriptions of layers and materials separated by a slash (/) indicate that the layers and materials are arranged in the order of the description.
As the electrical configuration of the memory cell array in an MRAM, generally a configuration is employed in which MTJ elements 1 are positioned at the intersections of bit lines 5 and write word lines 6 arranged in the form of a perpendicular matrix. Here, each MTJ element 1 is combined with a MOS transistor used as switching elements for cell switching to form one bit cell.
One writing method used in such MRAM entails passing current in both a bit line 5 and a write word line 6, applying the magnetic field induced by both the bit line 5 and the write word line 6 to the vicinity of the point of intersection, and causing reversal of the magnetization in the ferromagnetic free layer 2 by means of this magnetic field (hereafter this is called the “magnetic field writing method”). A circuit configuration for such addressing of MTJ elements is shown in FIG. 3. Here, a strong magnetic field is not obtained either from bit line 5 alone or from write word line 6 alone, and a magnetic field equal to or greater than the magnitude of the magnetic field necessary to reverse the ferromagnetic free layer 2 (the switching magnetic field) is not obtained; but at the position of intersection of both lines, at which the magnetic field created by the bit line 5 and the magnetic field created by the write word line 6 are both applied, a magnetic field equal to or greater than the switching magnetic field can be obtained, and so writing can be performed by selecting the bit line 5 connected to the desired bit cell and passing current, and also passing current through the write word line 6. In a readout operation of this method, first the desired bit line 5 and read word line 7 are selected. Then, by means of current flowing from the selected bit line 5, through the MTJ element 1, via the route to the readout electrode 8 and selected read word line 7, the voltage value occurring at the MTJ element 1 (that is, the resistance value of the MTJ element 1) is detected. Further, the state of the set of magnetizations at the MTJ element connected to the selected bit line 5 and read word line 7 is identified. By means of this processing, a readout operation is performed. Here, the intermediate value between the detected voltages for the case in which the magnetization directions are parallel and the case in which the directions are antiparallel in the MTJ element 1 is set as a reference voltage, and by means of the detected voltage or resistance value, it is possible to identify either a “1” or a “0” as having been allocated to the combination of magnetization directions of the ferromagnetic free layer 2 and ferromagnetic fixed layer 4 of the MTJ element 1.
However, when MRAM is manufactured with a high level of integration using MTJ elements configured so as to use the magnetic field writing method, as elements are made smaller, the switching magnetic field of the ferromagnetic free layer 2 increases, while the current passed through bit lines 5 and write word lines 6 decreases. Hence the problem arises that causing reversal of the magnetization of the ferromagnetic free layer 2, that is, writing data, becomes difficult, and this constitutes a major engineering issue for MRAMs.
As one means to resolve this, in place of the magnetic field writing method, MTJ elements utilizing technology in place of the magnetic field writing method in which spin-polarized current is passed spanning the ferromagnetic free layer and the ferromagnetic fixed layer to cause reversal of the magnetization of the ferromagnetic free layer (called “spin-transfer magnetization reversal”), as well as MRAMs using such elements, have been developed. FIG. 4 shows the configuration of MRAM utilizing spin-transfer magnetization reversal; this configuration is proposed in Patent Reference 2 and elsewhere.
When spin-transfer magnetization reversal is used, writing is performed as follows. First, the passing of a current so as to inject electrons from the ferromagnetic fixed layer 4 into the ferromagnetic free layer 2 is considered. The spin of electrons passing through the ferromagnetic fixed layer 4 is polarized in the magnetization direction of the ferromagnetic fixed layer 4 by the action of spin-torque from the magnetization of the ferromagnetic fixed layer 4 resulting from the exchange interaction with the magnetization, that is, the electrons are spin-polarized. When spin-polarized electrons enter the ferromagnetic free layer 2, they then impart a spin-torque to the magnetization of the ferromagnetic free layer 2. In this way, the magnetization of the ferromagnetic free layer 2 is arranged to be parallel to the magnetization of the ferromagnetic fixed layer 4. Conversely, when a current is passed such that electrons are injected from the ferromagnetic free layer 2 into the ferromagnetic fixed layer 4, electrons have spin antiparallel to the ferromagnetic fixed layer 4 are reflected at the interface between the ferromagnetic fixed layer 4 and the insulating layer 3, and the reflected electrons impart a spin-torque to the magnetization of the ferromagnetic free layer 2. As a result, the magnetization of the ferromagnetic free layer 2 becomes antiparallel to the magnetization of the ferromagnetic fixed layer 4. Based on the above principle, by selecting the direction of current applied to the multilayer film, the magnetizations of the ferromagnetic fixed layer 4 and ferromagnetic free layer 2 can be made parallel or antiparallel. This is the spin-transfer magnetization reversal technique.
In order to perform actual write operations using the spin-transfer magnetization reversal technique, a current larger than the current value necessary to perform magnetization reversal in the ferromagnetic free layer 2 by means of a current (the critical current) is necessary. During readout, a current smaller than the critical current is passed, and similarly to conventional MRAMs, the resistance value or voltage value is detected and data readout performed. In addition to the advantage of enabling writing even when integration levels are raised, MRAM utilizing the spin-transfer magnetization technique has the further advantage that, compared with conventional MRAM, write word lines to generate a write magnetic field are unnecessary, so that the element structure can be simplified.    Patent Reference 1: Japanese Patent Application Laid-open No. 2006-80116    Patent Reference 2: Japanese Patent Application Laid-open No. H11-120758    Non-patent Reference 1: J. Z. Sun, “Spin-current interaction with a monodomain magnetic body: A model study”, Physical Review B, volume 62, number 1, page 570, American Physical Society, 2000
However in MRAM which adopts the magnetic field writing method, and even in MRAM which adopts the spin-transfer magnetization reversal technique (spin-transfer magnetization reversal MRAM), there is the problem that the current necessary for writing is too large. Specifically, the critical current density necessary for magnetization reversal in spin-transfer magnetization reversal MRAM is still 106 A/cm2 or higher, and it is necessary to develop technology such that the critical current density is at the practical level of 105 A/cm2. In magnetic field writing MRAM as well, reduction of the current value necessary for writing is a common problem.
This invention was devised in order to resolve at least one of these problems.